1. Field of the Invention
The present invention relates to a method for recovering phosphorus from organic sludge generated at sewage treatment plants.
2. Description of Related Art
Organic sludge generated at sewage treatment plants is mostly transported to final disposal facilities as incinerated ash after being dehydrated and incinerated. However, it has become progressively difficult to secure final disposal facilities, and an effective use for incinerated ash has been desired.
Table 1 shows the composition of typical incinerated organic sludge ash (oxide equivalent, hereinafter the same). As is shown in the table, incinerated ash contains a large amount of phosphorus, which is a residue of the phosphorus contained in sludge obtained due to concentration by incineration. The phosphorus content in incinerated ash differs depending on the type of the coagulant added when sludge is precipitated, and polymeric incinerated sludge ash in which a polymeric coagulant is added has a larger amount of phosphorus in comparison with calcic incinerated sludge ash in which lime is added. In addition, incinerated sludge ash generated when sewage is dephosphorized contains a much larger amount of phosphorus, as shown in Table 2, and sometimes the phosphorus content is equal to that of a low-grade phosphorous ore.
TABLE 1 ______________________________________ (wt %) P.sub.2 O.sub.5 CaO Fe.sub.2 O.sub.3 Al.sub.2 O.sub.3 SiO.sub.2 ______________________________________ Polymeric incinerated 13.0 8.2 9.6 14.8 40.7 sludge ash Calcic incinerated 4.9 40.1 15.7 8.1 22.3 sludge ash ______________________________________
TABLE 2 ______________________________________ (wt %) P.sub.2 O.sub.5 CaO Fe.sub.2 O.sub.3 Al.sub.2 O.sub.3 SiO.sub.2 ______________________________________ Polymeric incinerated 21.9 10.1 3.1 12.2 32.3 sludge ash ______________________________________
In view of the effective use of incinerated ash containing such a large amount of phosphorus, various methods for recovering phosphorus from incinerated organic sludge ash have been disclosed. For example, in Japanese Unexamined Patent Publication No. 7-251141 (JP 7251141 A) and Japanese Unexamined Patent Publication No. 9-77506 (JP 9077506), a method for recovering phosphorus by acidolysis-solvent extraction is disclosed. In this method, an acid is added to incinerated ash to produce a slurry, phosphorus compounds in incinerated ash are decomposed to change phosphorus into soluble phosphoric acid, and phosphoric acid in the solution is extracted by an organic solvent after solid-liquid separation. In Japanese Unexamined Patent Publication No. 9-145038 (JP 9145038 A), a method for recovering phosphorus by melting is disclosed. In this method, incinerated ash with an added carbon source is heated and melted, phosphorus compounds in incinerated ash are reduced, and phosphorus is recovered by vaporization.
In accordance with the methods described in the Japanese Unexamined Patent Publication Nos. 7-251141 and 9-77506, however, processes such as acidolysis and solvent extraction must be combined, resulting in significantly complex treatment. Additionally, a large amount of residue is generated, for example, when incinerated ash is subjected to acidolysis to perform solid-liquid separation, and the disposal of the residue must be taken into consideration. On the other hand, in accordance with the method described in the Japanese Unexamined Patent Publication No. 9-145038, since incinerated ash must be melt by heating up to a significantly high temperature of 1,400.degree. C. or more, a large amount of energy is consumed and furnace maintenance costs increase.